Use _64 only if you have greater than 4 G of ram on the server and only if the processors support 64bit. If they don't support 64bit the install will tell you first thing, which helps you to not waste time.

I would figure out if the processors support 64bit first before even installing more than 4G of ram.

Use _64 only if you have greater than 4 G of ram on the server and only if the processors support 64bit.

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Are you completely insane? :D

What I mean by that is that you DO NOT need to have 4 GB in order to install and gain the performance advantages of 64 bit!

For some odd reason, it is a common misconception that 64 bit is for meant only for systems greater than 4 GB of memory but the truth is that 64 bit just simply SUPPORTS servers that have greater than 4 GB of memory. In other words, on systems less than 4 GB, you have a "choice" to make and can go with either 32 bit or 64 bit but after you reach 4 GB, you MUST go with 64 bit else the full address space of your memory won't be accessible or usable by the operating system. That's it, nothing more! You do not need 4 GB of memory to be able to install or run 64 bit!

However, you do need a processor that supports 64 bit! Today though, nearly all modern processors fully support 64 bit and the performance advantages of running 64 bit are fairly significant. The software support and incompatible problems of old are long since gone. Subsequently, there is really no reason left to stay running 32 bit unless you are simply running a very old server with an obsolete processor. However, if that is the case, you might consider upgrading your server's hardware soon anyway.

Bottom line is just about everyone these days can use 64 bit and it will improve your performance significantly over running 32 bit on the same system so unless you have a really old machine, I would absolutely go with 64 bit!

CentOS 5.3 64 bit also works beautifully with Cpanel / WHM and is personally my preferred platform of choice for Cpanel based servers.

And I purposely get servers with 64bit processors and tons of ram. I did recently attempt a 64bit install on an old intel server with dual 2.8 Xeons to find out they were not 64bit compatible. CentOS told me immediately.

So allow me to slightly rephrase- you will certainly want _64 if you plan on using more than 4G of ram.

And I purposely get servers with 64bit processors and tons of ram. I did recently attempt a 64bit install on an old intel server with dual 2.8 Xeons to find out they were not 64bit compatible. CentOS told me immediately.

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Thus my earlier discussion about having "older" processors.

Wow if it was really a 2.8 Xeon and not supporting 64 bit, it must have been one of the few very early Xeons that came out during the transition period because all the Xeons after that fully support 64 bit. :D

In contrast 32-bit applications can only address 4 GB of memory space, unless specially coded to address more on PAE systems.

32-bit applications on a 64-bit operating system can still only address 4 GB of memory space, unless specially coded to address more on PAE systems. The memory model on x86-64 is an extension of PAE.

64-bit applications will often use more memory than their 32-bit equivalents, due to the basic size difference in 32-bit vs. 64-bit. This is often where the notion of "only use a 64-bit OS with 4+ GB RAM" stems from.